Ground Plan of the Colosseum
"Ground Plan of the Colosseum at Rome, with representation of the Tiers of Seats, and of the Substructure. The shape of the amphitheatre was borrowed from that of the theatre. Since no stage was necessary, and with a view to securing as many rows of seats as possible, the semicircular form of the theatre was done away with, and the whole became a circle, or rather an ellipse, which was afterwards the recognized shape for all buildings of this nature, whether at Rome or in the provinces."
A. Rosengarten, W. Collett-Sandars A Handbook of Architectural Styles (New York, NY: Charles Scribner's Sons, 1895)